Image pickup device comprising photoelectric conversation unit, floating diffusion region and guard ring

ABSTRACT

The present invention uses an image pickup device comprising a plurality of pixels respectively including a photoelectric conversion unit for converting incoming light into a signal charge, an amplifying unit for amplifying the signal charge generated by the photoelectric conversion unit and a transfer unit for transferring the signal charge from the photoelectric conversion unit to the amplifying unit, in which the photoelectric conversion unit is formed of a first-conductivity-type first semiconductor region and a second-conductivity-type second semiconductor region and a second-conductivity-type third semiconductor region is formed on at least a part of the gap between a photoelectric conversion unit of a first pixel and a photoelectric conversion unit of a second pixel adjacent to the first pixel, a first-conductivity-type fourth semiconductor region having an impurity concentration higher than that of the first semiconductor region is formed between the photoelectric conversion unit and the third semiconductor region and a first-conductivity-type fifth semiconductor region formed at a position deeper than the fourth semiconductor region and having an impurity concentration higher than that of the first semiconductor region is included between the photoelectric conversion unit and the third semiconductor region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup device for convertingan image to an electrical signal, and more particularly, to an activepixel sensor (hereafter referred to as APS) having an amplifier for eachpixel or several pixels.

2. Related Background Art

An image pickup device serving as an APS is also referred to as a CMOSsensor and widely used for digital cameras. This conventional imagepickup device is described below by referring to FIGS. 9 to 12.

FIG. 9 is a top layout drawing pf pixels of a conventional image pickupdevice.

FIG. 10 is a cross-sectional structural drawing along the line 10-10 inFIG. 9.

FIG. 11 is a concentration profile along the line Y-Y in FIG. 10.

FIG. 12 is an equivalent circuit diagram using the image pickup devicein FIG. 9.

In FIG. 12, reference numeral 121 denotes a photodiode (PD) forconverting light into a signal charge and 122 and others denote aprocessing circuit for driving a PD or processing the signal charge ofthe PD 121. Reference numeral 122 denotes a transfer MOS transistor fortransferring a signal charge generated by a photodiode, 123 denotes afloating diffusion (FD) region for temporarily storing the transferredsignal charge, 124 denotes a resetting MOS transistor for resetting thefloating diffusion region 123 and PD 121, 125 denotes a source followerMOS transistor for converting the signal charge of the floatingdiffusion region 123 into a voltage and amplifying the voltage by asource follower amplifier, 126 denotes a selection MOS transistor forselecting an optional one row in an array and 127 denotes a read linecommonized by one column to read a pixel voltage signal. Pixels arearranged like an array to constitute an image pickup device.

In FIGS. 9 and 10, a PD of a photoelectric conversion unit isconstituted of a P-type well region (hereafter referred to as “PWL”) 102and a PN junction in an N region 103 on an N-type substrate 101.Reference numeral 107 denotes an N+ region serving as a source-drainregion of a source follower MOS transistor which is a part of a circuitfor amplifying a signal charge generated by a PD.

In the case of the APS of a prior art, the N region 103 of a PD servingas a photoelectric conversion unit is one of large sources forrespectively generating a dark current. A dark current componentgenerated nearby a separation layer 105 on the fringe of the N region103 is present in addition of a component generated on the entiresurface of the N region 103. Therefore, the N region 103 of the PD isseparated from the separation layer 105 which is a generation region ofthe dark current component by a certain distance. A P+ region (channelstop region) 104 having a high P-type impurity concentration is formedbetween the separation layer 105 and the N region of the PD in order notto make a depletion layer extending from the N region 103 of the PDreach the separation layer. An example of the configuration is disclosedin Japanese Patent Application Laid-Open No. H10-308507. Moreover, inU.S. 2003160295A, it is reported that a dark current is extremelygenerated under a poly-wiring on a separation layer. The channel stopregion 104 raises a P-type concentration of a channel stop region undera separation layer in which a poly-wiring (or poly-gate) 106 is set andrealizes an effect for restraining a dark current by forming aconcentration barrier on a path to the N region 103.

However, even by forming the channel stop region, when deepening aconcentration profile of a PWL and using a PWL structure for improving aquantum efficiency of photoelectric conversion by a PD, a new technicalproblem is found that a dark current increases.

Specifically, as shown in FIG. 11, a dark current increases at a rate ofmore than 10% by changing a concentration profile of PWL₁ 102-1 to PWL₂102-2. Moreover, by setting a profile of PWL₃ 102-3, the dark currentincreases at a rate of tens of percents. This is a phenomenon whichcannot be explained by a conventional idea that a dark current isgenerated because a depletion layer contacts with a separation layer.Moreover, when a well is deepened, a problem further occurs that thenumber of charges leaked from adjacent pixels increases. Specifically, aproblem of deterioration of a blooming phenomenon or color mixtureoccurs.

Therefore, it is an object of the present invention to greatly restraina dark current, blooming and color mixture when the concentrationprofile of the PWL is deep and a PWL structure for improving a quantumefficiency of photoelectric conversion by a PD is used.

SUMMARY OF THE INVENTION

To solve the above problems, an amplifying type image pickup device ofthe present invention comprises a plurality of pixels each having aphotoelectric conversion unit, a plurality of amplifying elements eachprovided for one or more photoelectric conversion units, and a MOStransistor for reading signals from the photoelectric conversion units,the pixels being arranged on a second-conductivity-type semiconductorsubstrate, wherein

the photoelectric conversion unit comprises a first-conductivity-typefirst semiconductor region and a second-conductivity-type secondsemiconductor region so as to form a PN junction between the first andsecond semiconductor regions,

an element isolation region formed by an insulator for isolating thephotoelectric conversion unit from an element adjacent to thephotoelectric conversion unit is arranged,

a second-conductivity-type third semiconductor region forming anelectrode region of the MOS transistor through the element isolationregion is arranged between first and second ones of the photoelectricconversion units adjacent to each other,

a wiring for controlling a gate potential of the MOS transistor isarranged on the element isolation region arranged between the first oneof the photoelectric conversion units and third semiconductor region,

a first-conductivity-type fourth semiconductor region having an impurityconcentration higher than the impurity concentration of the firstsemiconductor region and a first-conductivity-type fifth semiconductorregion arranged deeper than the fourth semiconductor region withreference to an element forming plane of the semiconductor substrate arearranged under the element isolation region adjacent to the first one ofthe photoelectric conversion units, and

the fifth semiconductor region has an impurity concentration higher thanan impurity concentration of the first semiconductor region, and a widthlarger than a width of the fourth semiconductor region, and

the third semiconductor region is disposed within a sixth semiconductorregion of an impurity concentration lower than impurity concentration ofthe fourth and fifth semiconductor regions.

In addition, an amplification type image pickup device of the presentinvention comprises a plurality of pixels each having a photoelectricconversion units, a plurality of amplifying elements each provided forone or more photoelectric conversion unit and a MOS transistor forreading signals from the photoelectric conversion units, the pixelsbeing arranged on a second-conductivity-type semiconductor substrate,wherein

the photoelectric conversion unit comprises a first-conductivity-typefirst semiconductor region and a second-conductivity-type secondsemiconductor region so as to form a PN junction between the first andsecond semiconductor regions,

an element isolation region formed by an insulator for isolating thephotoelectric conversion unit from an element adjacent to thephotoelectric conversion unit is arranged,

a second-conductivity-type third semiconductor region forming anelectrode region of the MOS transistor through the element isolationregion is arranged between first and second ones of the photoelectricconversion units adjacent to each other,

a first-conductivity-type fourth semiconductor region having an impurityconcentration higher than the impurity concentration of the firstsemiconductor region and a first-conductivity-type fifth semiconductorregion arranged deeper than the fourth semiconductor region withreference to an element forming plane of the semiconductor substrate arearranged under the element isolation region adjacent to the first one ofthe photoelectric conversion units, and

wherein the third semiconductor region is arranged in afirst-conductivity-type sixth semiconductor region having an impurityconcentration lower than the impurity concentration of the fourth andfifth semiconductor regions.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top layout view of pixels of an image pickup device ofEmbodiment 1 of the present invention;

FIG. 2 is a sectional structural view along the line 2-2 in FIG. 1 ofthe present invention;

FIG. 3 is a sectional structural view of pixels of an image pickupdevice of Embodiment 2 of the present invention;

FIG. 4 is a top layout view of pixels of an image pickup device ofEmbodiment 3 of the present invention;

FIG. 5 is an illustration showing paths in which dark current, colormixture and blooming occur;

FIGS. 6A and 6B are concentration profiles of a PWL of the Embodiment 1;

FIG. 7 is a sectional structural view of pixels of the image pickupdevice of the Embodiment 2;

FIGS. 8A, 8B and 8C are concentration profiles of a PWL and P+ guardlayer of Embodiment 3;

FIG. 9 is a top layout view of pixels of an image pickup device of aprior art;

FIG. 10 is a sectional structural view along the line 10-10 in FIG. 9;

FIG. 11 is a concentration profile along the line Y-Y in FIG. 10;

FIG. 12 is an equivalent circuit diagram of a pixel used for FIG. 9;

FIG. 13 is a sectional structural view (i) of a pixel for explaining aproblem of a prior art;

FIG. 14 is a sectional structural view (ii) of a pixel for explaining aproblem of a prior art; and

FIG. 15 is an illustration showing a circuit block when applying animage pickup device of the present invention to a camera;

FIGS. 16A, 16B and 16C are concentration profiles of a PWL and P+ guardlayer of Embodiment 5; and

FIG. 17 is a top layout view of pixels of an image pickup device ofEmbodiment 5 of the present invention.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the principle and means of the present invention are describedbelow in detail. As a result of earnestly studying the above problems,the present inventor et al. found the following solving means.

It is useful to set the above P-type semiconductor region for channelstop slightly separately from a separation layer at the MOS transistorside because the impurity concentration of the region is high. When theP-type semiconductor region is formed by contacting with the separationlayer at the MOS transistor side, a narrow channel phenomenon occurs,the effective channel width of the MOS transistor decreases and it isdifficult to form a fine pixel. Therefore, a region free from P-typesemiconductor region is left even under the poly-wiring 106 as shown inFIG. 10 due to the problem of the layout. Conventionally, it has beenconsidered that most of the current generated in this region isdischarged to the N+ region 107 of the MOS transistor by passing throughthe path 2. However, it is found that some of the current is dischargedto the N-type substrate 101 by the path 3 and some of the current isdischarged to the N region 103 of the PD by passing through the downsideof the P-type semiconductor region. Particularly, in the case of a PWLstructure having a deep concentration profile, a relation between a PWLstructure and a dark current that the dark current is increased by apath 4 is found.

More minute description is made. For example, among electrons generatedin the poly-wiring 106, 100 (unit is optional) flow through paths 3 and4 other than the path 2. This component is superimposed on the darkcurrent component generated on the entire surface. However, in the caseof the concentration profile of PWL₂ 102-2, 80 flows through the path 3and remaining 20 flows through the path 4. As a result, the dark currentcomponent increases. In the case of the structure of PWL₃ 102-3,however, because a concentration barrier is formed in a deep portion,the path 3 is greatly decreased and as a result, the path 4 may increaseup to approx. 50.

Therefore, in the case of an image pickup device of the presentinvention having a plurality of pixels including a photoelectricconversion unit for converting incoming light into a signal charge, anamplifying unit for amplifying the signal charge generated by thephotoelectric conversion unit and a transfer unit for transferring thesignal charge from the photoelectric conversion unit to the amplifyingunit, the photoelectric conversion unit is formed of afirst-conductivity-type first semiconductor region, and asecond-conductivity-type second semiconductor region, asecond-conductivity-type third semiconductor region is formed on atleast a part of the portion between the photoelectric conversion unit ofa first pixel and the photoelectric conversion unit of a second pixeladjacent to the first pixel, a first-conductivity-type fourthsemiconductor region having an impurity concentration higher than thatof the first semiconductor region is formed between the photoelectricconversion unit and the third semiconductor region and formed at aposition deeper than the fourth semiconductor region between thephotoelectric conversion unit and the third semiconductor region, and afirst-conductivity-type fifth semiconductor region having an impurityconcentration higher than that of the first semiconductor region isincluded.

In this case, “deep” is a degree of a distance from a principal planeserving as a photoreceiving surface of an image pickup device to anotherprincipal plane facing the former principal plane. Moreover, “horizontaldirection” is a direction orthogonal to the depth direction.

Hereafter, specific embodiments of the present invention are described.However, a portion common to that of a conventional device is providedwith a same symbol.

Embodiment 1

FIG. 1 is a top layout view of pixels of the image pickup device of theEmbodiment 1 of the present invention.

FIG. 2 is a sectional structural view along the line 2-2 in FIG. 1.

In FIG. 2, reference numeral 204 denotes a P-type semiconductor region(P+ guard layer) peculiar to the Embodiment 1.

As shown in FIG. 2, the present invention greatly decreases the darkcurrent in accordance with a model for generating the dark current byusing the following structure. As such configuration, (1) the P-typesemiconductor region 204 is made to extend up to a position nearby theN-type substrate 101. Moreover, it is allowed to use a structureincluding at least a second P-type semiconductor region 204-2 at aposition deeper than a first P-type semiconductor region 204-1.Furthermore, it is allowed to include a third P-type semiconductorregion 204-3.

Furthermore, a configuration is used in which (2) the N+ region 107 isformed between the semiconductor region 204 and the semiconductor region204 of an adjacent pixel. The N+ region 107 is an N+ region serving as asource-drain region of a source-follower MOS transistor or the like usedfor amplification of a signal charge. In this case, an amplifying unitis formed between pixels.

In this case, a P-type semiconductor region and N+ region are used.However, it is enough that a P-type semiconductor region is an impurityregion having a conductivity type reverse to that of the region shown by103 for forming a photoelectric conversion unit and storing charges.Moreover, it is enough that an N+ region is an impurity region having aconductivity type reverse to that of the impurity region. In this case,in FIG. 1, a plurality of pixels (2×2=4 pixels in FIG. 1) are formed.

The structure of (1) restrains a flow to the path 4 in FIG. 2 and thestructure of (2) accelerates a flow to the path 2 to resultantlydecrease the flow to the path 4.

Particularly as shown in FIG. 11, in the case of the PWL concentrationprofile PWL₃ 102-3 (concentration profile is deeper than a conventionalone or a concentration peak is present at a deep position (little gradewell structure)), the path 3 is greatly restrained. Therefore, thenumber of cases in which electrons which must originally flow throughthe path 3 flows through the path 4 is increased. However, it ispossible to greatly restrain the path 4 by a configuration having notonly the structure (1) but also the structure of (2).

FIGS. 13 and 14 are sectional structural views (i) and (ii) of pixelsfor explaining problems of a prior art.

As shown in FIG. 14, unless an N+ region is formed in a deviceseparation portion at the right side of the right pixel, the rate ofelectrons which finally flow into the N region 103 of a photodiodeincreases. Moreover, even if the P-type semiconductor region at thecenter of FIG. 14 is set like 104-1, it is impossible to prevent flowinginto the N region 103 of the photodiode because there is no sealingeffect though there is a certain effect.

Then, a phenomenon in which the number of charges leaking from adjacentpixels is increased is described below by referring to FIGS. 13 and 14.

In the case of a shallow PWL structure, it is not necessary to considerthe path 6 in FIG. 13 but the path 5 is mainly considered. Because thepath 5 is attracted into an N+ region by setting the N+ region, theblooming phenomenon and color mixture are very small.

However, by using a deep PWL structure, a rate at which the path 6 isformed increases. Particularly, by using the structure of the PWL₃ 102-3in FIG. 11, a path of dark current to the N-type substrate 101 isrestrained because there is a concentration barrier. Therefore, chargesflowing into adjacent pixels by passing through the path 6 increases. Inthe case of the above structure of (1), the P-type semiconductor regionserves as a concentration barrier (potential barrier) when charges flowinto adjacent pixels to restrain leak of the charges into adjacentpixels. Description is made below by referring to FIG. 14. In the caseof the structure shown at the central portion of FIG. 14, theversatility for setting a concentration is lowered because a MOStransistor also serves as a well through the P-type semiconductor region104-1 has the function of restraining charges from flowing into adjacentpixels.

However, to leak into adjacent pixels by a structure in which a P wellfor providing a channel unit of a MOS transistor for forming pixels isheld by the P-type semiconductor regions 204, it is necessary to exceedtwo concentration barriers. Moreover, before exceeding the concentrationbarriers (potential barriers) of the P-type semiconductor region 204 ofadjacent pixels, the frequency of exceeding the concentration barriers(potential barriers) of the P-type semiconductor 204 is greatlyrestrained because the N+ region 107 is present and dark current isabsorbed into the N+ region 107. In this case, source-drain region of atransistor is formed and the impurity concentration of a P well forproviding the channel region of the source-drain region is set to a lowvalue compared to that of the P-type semiconductor region.

Embodiment 2

FIG. 3 is a sectional structural view of a pixel of an image pickupdevice of Embodiment 2.

The structure shown in FIG. 3 has a P-type semiconductor region 304-1set over the lower portion of a MOS transistor in an adjacentphotoelectric conversion unit under a P-type semiconductor region 304.In the case of the P-type semiconductor region 304-1, a concentration isset separately from the well of the MOS transistor and it is possible toform a high concentration barrier (potential barrier). It is possible toaccelerate discharge of a few charges exceeding the concentrationbarrier (potential barrier) to the N+ region 107 and further restrainthe charges from leaking into adjacent pixels by optimizing theconcentration profile of the P-type semiconductor region 304-1.

Embodiment 3

FIG. 4 is a top layout view of pixels of an image pickup device ofEmbodiment 3.

As shown in FIG. 4, by forming a P-type semiconductor region 404 into anannular shape and setting the region so that a photoelectric conversionunit and charge voltage conversion unit (FD) are included in the region,it is possible to provide an image pickup device in which dark current,blooming and color mixture are further restrained.

FIG. 5 is an illustration showing paths generated due to dark current,color mixture and blooming.

As shown in FIG. 5, when setting a P-type semiconductor region 504 asdescribed above, paths 4-1 and 6-1 are left and a slight dark current,blooming and color mixture are left. However, by using the structureshown in FIG. 4, the paths 4-1 and 6-1 are restrained by the P-typesemiconductor region 404 and it is possible to use the structure as adischarge destination of excessive charges and dark-current charges bysetting a charge voltage conversion unit (FD) in a region covered withthe P-type semiconductor region 404. Moreover, by setting this portionto a fixed voltage during a storage period, it is possible to dischargemany charges. When there is no discharge port, paths are restrained by aconcentration barrier (potential barrier). However, when charges areexcessively generated, they exceed the concentration barrier and flowinto adjacent pixels. By setting the discharge port, excessive chargesare discharged and it is possible to greatly restrain the paths 4-1 and6-1.

That is, it is important for the present invention to set not only aconcentration barrier but also a charge discharge port and use astructure in which excessive charges easily flow into the dischargeport.

EXAMPLE 1

This example has a configuration in which a photoelectric conversionunit and an amplifying unit are set in the same PWL.

FIGS. 6A and 6B are concentration profiles of PWLs of this example.

The middle-concentration profile PWL₁ 602-1 in FIG. 6A has aconventional PWL structure and the PWL₂ 602-2 in FIG. 6A has a PWLstructure of this example. By deepening the structure compared to aconventional well structure, it is possible to capture charges generatedin a deeper portion by a PD and thereby improve sensitivity.

A problem of dark current or color mixture influences a performance likethe present invention in the case of a visible-light sensor mounted on adigital camera or video. The visible-light sensor is designed by puttingweight on the visual sensitivity of a person and preferably, the peak ofa spectral sensitivity in a semiconductor layer is set so that awavelength becomes equal to approx. 550 nm. That is, it is ideal thatthe optical signal divide shown in FIG. 6A becomes approx. 4 μm.

Therefore, the present invention is particularly effective for avisible-light sensor because of its performance, the depth of a PWL isideally 4 μm and a range of 1 to 6 μm is a target of its substantialscope.

FIG. 6B is a concentration profile a P-type semiconductor region formedunder a device separation region which is a characteristic requirementof the present invention. Because a PWL structure becomes deep, a firstP-type semiconductor region 604-1 does not reach the “boundary depth(optical signal divide) in which optical charges flow to the N region103 shown in FIG. 6C. Therefore, in the case of this example, a secondP-type semiconductor region 604-2 is formed.

To extend the first P-type semiconductor region up to “C”, spread in thehorizontal direction becomes a problem and it is difficult tominiaturize pixels. The N region of a PD is substantially decreased bythe influence of the horizontal spread. Therefore, in the case of thisexample, a second P-type semiconductor region is formed by ionimplantation energy different from formation of the first P-typesemiconductor region at a position deeper than the first P-typesemiconductor region.

As a result, dark current is improved at ratios shown in the followingTable 1.

TABLE 1 First P+ Second P+ Dark current PWL structure guard layer guardlayer (Ratio) PWL₁ Present Absent 1 PWL₂ Present Absent 2 PWL₂ PresentPresent 1.2

Thus, according to this embodiment, by deepening a PWL and therebyincreasing the number of carriers which can be captured by a photodiode,it is possible to lower dark current up to a level almost equal to thecase in which the PWL is shallow.

EXAMPLE 2

FIG. 7 is a sectional structural view of a pixel of an image pickupdevice of Example 2.

A case of forming a first P-type semiconductor region 704 and a secondP-type semiconductor region 704-1 and a case of forming a first P-typesemiconductor region 704 and a second P-type semiconductor region 704-1and a third semiconductor region 704-2 in FIG. 7 are described below.

The feature of this example is that horizontal widths of the FirstP-type semiconductor region 704, second P-type semiconductor region704-1 and third P-type semiconductor region 704-2 are kept in a relationof 704<704-1<704-2. Preferably, the second P-type semiconductor regionreaches “C” as described for the Example 1. However, even if the secondP-type semiconductor region does not reach “C”, a characteristic iseffectively improved by forming the second P-type semiconductor regionin a process (including a different ion implanting condition) separatefrom the case of the first P-type semiconductor region. The P-typesemiconductor region 704 serving as the first P-type semiconductorregion contacts with the N region of a MOS transistor or PD. Therefore,the horizontal width and the concentration of the region 704 arerestricted and thereby, it is impossible to take the width andconcentration sufficiently largely.

Compared to the case of extending the first P-type semiconductor region704 in the state having the above restriction in a deep direction, thesecond P-type semiconductor region 704-1 is formed separately from thefirst P-type semiconductor region 704 (formed in a separate process).Thereby, it is possible to set a concentration and a width withoutdeteriorating the performance of the above-described MOS transistor orphotodiode. That is, it is possible to set the concentration of thesecond P-type semiconductor region 704-1 highly than the concentrationof the P-type semiconductor region 704 and the horizontal width of theregion 704-1 widely than the horizontal width of the region 704. A widthcan be widened by using a separate mask and thereby forming asemiconductor region. Moreover, when using the same mask as the case ofthe first P-type semiconductor region 704, it is enough to form thesecond P-type semiconductor region 704-1 by using a diagonal ionimplantation method.

As a result, dark current and color mixture rate are improved at theratios shown in Table 2.

TABLE 2 PWL Dark current Color mixture structure 704 704-1 704-2 (Ratio)rate PWL₁ Present Absent Absent 1 0.5% PWL₂ Present Absent Absent 2 1.5%PWL₂ Present Present Absent 1.1 0.5% PWL₂ Present Present Present 0.70.2%

Moreover, it is preferable that the third P-type semiconductor region704-2 is also set under an amplifying unit. Furthermore, the region704-2 extends up to a part of the lower portion of the N region of a PD.

This example has a structure capable of setting optical charges presentat a deep position to the N region 103 of the PD and correctly capturingor discharging charges not used for forming an image of dark current orcolor mixture charge from or to the N+ region 107. Moreover, thestructure is preferable from the viewpoint of fabrication. That is, whenforming a P-type semiconductor region at a deep position and using it asa barrier, it is necessary to implant ions at a high energy. In thiscase, however, it is necessary to thicken a resist. As a result, aresolution capacity is inevitably deteriorated and fine resolution isdifficult. By using the structure of this example, it is possible toavoid the above problem of fabrication.

The effect is confirmed even by the structure shown in FIG. 3. That is,it is possible to sufficiently obtain the effect of even the structurein FIG. 3. However, as described above, by applying the structure inFIG. 7, it is possible to obtain a larger effect. Moreover, it ispossible to widen its scope.

EXAMPLE 3

FIGS. 8A to 8C are concentration profile diagrams of a P-typesemiconductor region functioning as a barrier to a signal charge, formedbetween N+ regions present between photoelectric conversion unitsadjacent to a PWL and a photoelectric conversion unit of Example 3.

FIG. 8A shows a concentration profile of the PWL structure of thisexample. The PWL is fabricated by repeatedly implanting a plurality ofions having energies different from each other. PWL₃ 802-3 and PWL₄802-4 are formed of a plurality of wells (semiconductor regions)respectively having an impurity concentration peak.

FIG. 8B shows a concentration profile of a P-type semiconductor regionfunctioning as a barrier. The profiles are formed in accordance with theimplanting energy condition same as the case of the PWL. The fabricationprocess is formed in accordance with the LOCOS method in order to form aseparation layer after implanting ions into the PWL and P-typesemiconductor region. Reference numerals 804-1 to 804-4 show first tofourth P-type semiconductor regions. A thin line in FIG. 8B shows the802-4 PWL₄ in FIG. 8A.

The concentration profiles in FIG. 8C are obtained by forming aseparation layer by the LOCOS method and then, implanting ions into aPWL and P-type semiconductor region though an implanting energycondition same as that in FIG. 8B is used. In any case, a preferablecharacteristic is obtained in which dark current, blooming and colormixture are greatly restrained. Particularly in FIG. 8C, in profiles ofPWLs, an extreme value is present in concentration and it is possible toraise a concentration barrier of a minimum value portion in whichcharges easily stay.

Moreover, in the case of this example, the fourth P-type semiconductorregion 804-4 does not reach an optical-signal divide. However, as shownin FIGS. 8A to 8C, optical charges generated in a region in which aP-type semiconductor region is insufficient immediately flow into a PWLhaving a low concentration because of their concentration gradient.Therefore, leak to adjacent pixels at this portion is sufficiently smalland it is possible to confirm that even the structure shown for thisexample has a sufficient restrain effect.

This example has one more characteristic. By forming a PWL and a P-typesemiconductor region in accordance with a plurality of ion implantationsteps, it is possible to select a proper concentration at each depth andderive a higher sensor performance. Specifically, a P-type impurityregion having a high concentration present at the lower portion of the Nregion 103 of the photodiode in FIG. 8A is used. It is preferable thatthe impurity region has a concentration of 5E16 cm⁻³ or more in order toobtain a large handling charge value. Moreover, it is preferable that aP-type impurity region located at the deepest position has aconcentration of 1E17 cm⁻³ or more in order to improve the sensitivity.Moreover, it is preferable that a P-type impurity region present betweenthem has a concentration of ⅕ to 1/10 in order to flow signal chargesinto a PD compared to the concentration of a P-type impurity regionlocated as the deepest position.

Moreover, to improve the sensor performance, it is preferable tosimilarly design a P-type semiconductor region correspondingly to adesigned PWL structure. Setting second and third P-type semiconductorregions independently of the first P-type semiconductor region like thecase of this example is particularly effective for improvement of theperformance of a sensor having a plurality of wells respectively havingan impurity concentration peak and formed at different depths. In thiscase, the PWL and the P-type semiconductor region functioning as abarrier are formed at almost the same depth.

An approximate value of each concentration is described by referring toFIGS. 8A to 8C. In FIGS. 8A to 8C, PWLs have surface concentrations of1E16 to 3E17 cm⁻³ because it is necessary to be coexistence with atransistor. The concentration of a PWL at the deepest position is up to1E18 cm⁻³ and an intermediate PWL has a concentration smaller than thatof the PWL at the deepest position, preferably ½ or less. A trend ofimprovement is obtained when the concentration is not ½ or less. Theabove structure makes it possible to more efficiently collect opticalcharges in a PD. Moreover, the guard region is preferably more effectivewhen the region has a concentration two times or more higher than thatof each PWL. However, by raising a concentration, a trend of improvementis obtained.

EXAMPLE 4

This example corresponds to the Embodiment 3 in FIG. 4.

A concentration profile is prepared in the same condition as that inFIG. 8C.

As a result, dark current and the like are improved at ratios shown inTable 3.

TABLE 3 Dark Color Blooming Deep guard current mixture quantity Pixellayout layer (Ratio) rate (Ratio) Layout in FIG. 1 Absent 3   2% 3Layout in FIG. 1 Present 0.7 0.2% 1 Layout in FIG. 3 Present 0.6 0.1%0.5

In the case of this example, by using the pixel layout shown in FIG. 4,it is possible to get the paths shown in FIG. 5 under control andfurther improve the sensor characteristic compared to a pixel having thedeep P-type semiconductor region in FIG. 1. The deep P-typesemiconductor region shown in the table is at least one of 804-2 to804-4 formed in ion implantation steps of a plurality of times.

[Application to a Camera Body]

FIG. 15 is an illustration showing a circuit block when applying animage pickup device of the present invention to a camera.

There is a shutter 1001 before a photographing lens 1002 to controlexposure. A light intensity is controlled by a diaphragm 1003 accordingto necessity to image an object on an image pickup device 1004. A signaloutput from the image pickup device 1004 is processed by an image pickupsignal processing circuit 1005 and converted from an analog signal intoa digital signal by an A/D converter 1006. The output digital signal isfurther arithmetic-processed by a signal processing unit 1007. Theprocessed digital signal is stored in a memory unit 1010 and sent to anexternal unit through an external I/F unit 1013. The image pickup device1004, image-pickup signal processing circuit 1005, A/D converter 1006and signal processing unit 1007 are controlled by a timing generationunit 1008 and the whole system is controlled by a whole-control unit andarithmetic-operation unit 1009. To record an image in a recording medium1012, an output digital signal is recorded through a recording-mediumcontrol I/F unit 1011 controlled by the whole-control unit andarithmetic operation unit.

EXAMPLE 5

The effect described for the Example 3 is more minutely described belowby using the present example.

FIGS. 16A to 16C are illustrations for explaining this example. In thecase of this example, the concentration profile of a deep guard ring isdescribed. A reference numeral 1608 denotes p+region. A referencenumeral 1603 denotes N type region forming a photodiode. A referencenumeral 1604-4 denotes a concentration profile of a guard ring. Areference numeral 1601 denotes a concentration profile under aphotodiode.

Optical charges generated at a deep position by incoming light almostisotropically expand by diffusion transport when a concentration isunlimitedly uniform. That is, because the generated optical charges areexcessive charges, they isotropically expand in order to uniform theexcessive carriers. In fact, however, the photodiode side is depleteddue to the PN junction of the photodiode and an electron concentrationis low. Therefore, the current quantity by diffusion transport isincreased due to the electron concentration difference. Therefore, mostoptical carriers reach the photodiode but several % of them leaks toadjacent pixels. To restrain the leak of them, it is necessary to makeoptical carriers easily flow to the photodiode. Therefore, in the caseof this example, a potential gradient is formed by providing a gradientto impurity concentrations so as to flow to the photodiode side by theelectric field. Diffusion current is D*[dn/dx], D is a diffusionconstant, n is a carrier concentration, drift current is μ*n*E, and E isan electric field.

Moreover, when considering that a general carrier generation quantity bylight is up to 100,000 electrons/pixel/sec, a sufficient effect isobtained by setting an impurity concentration gradient of approx. onedigit.

In this case, an important idea is shown in FIGS. 16A to 16C.

In FIGS. 16A to 16C, a continuous line 1601 shows a concentrationprofile under a photodiode and a broken line 1604-4 is a concentrationprofile of a guard ring of the present invention.

In the case of the structure in FIG. 16A, though color mixture is almostrestrained, a small quantity flows into adjacent pixels through theregion F.

To further restrain color mixture, it is necessary to deepen the guardring up to the vicinity of the position C like FIG. 16B. To realize theabove mentioned, it is necessary to perform ion-implantation to theguard ring at a high energy. That is, an advanced technique is necessarywhich requires resist having a large film thickness and minutepatterning.

This example uses a structure in which concentration differences areconcentrated on a deep position, a concentration gradient of anintermediate layer is decreased by a value equivalent to concentrationof concentration differences, and color mixture in the region isrestrained by a deep guard ring like FIG. 16C.

That is, the concentration profile of the photodiode is roughly dividedinto three regions as shown below.

-   <1> Deep region having a large concentration gradient (Preferably, 1    digit/1.5 μm)-   <2> Intermediate region having no concentration gradient or a small    concentration gradient-   <3> Shallow region in which photodiode is present

As a result, diffusion in cross direction is restrained by a potentialgradient at a deep portion and an intermediate layer can restrain colormixture by a guard ring. Specifically, C is set to 3.5 μm and formedthrough ion implantation at approx. 2.5 MeV of boron, and a peakconcentration is set to approx. 2e17 cm⁻³. An intermediate region isrealized at a position of approx. 2 μm and a deep guard ring is formedthrough ion implantation of approx. 1.5 MeV of boron.

As a result, a high color-mixture restraining force can be obtained byapplying comparatively easy middle-degree ion implantation to the deepguard ring.

Then, in the case of the concentration of a guard ring in anintermediate region, it is possible to restrain up to color mixture ofapprox. 1% or less by having a concentration at least approx. 5 times,preferably 10 times higher for a width of 1 μm of the guard ring to theconcentration under a photodiode. Particularly as one characteristic ofa deep guard ring of the present invention, a wide guard ring region isformed. This deep guard ring can be set under a MOS transistor servingas a surface device. This is because a higher restraint effect can beobtained when the deep guard ring has the same concentration as aconventional shallow-position guard ring and it is possible to restraina concentration in order to restrain a damage to a white defect ortransistor for the same restraint effect.

It is important for the present invention that a higher color-mixturerestraint effect can be obtained by forming a deep guard ring under aMOS transistor or electrode. However, the electrode is not indispensableas a sectional structure but it is enough that an N+ electrode is set toa part of the sectional structure. Only setting a wide guard ringprovides a color-mixture prevention effect. As shown in FIG. 17, thougha region A or region B is a region where an electrode of a MOStransistor is not formed, a color-mixture prevention effect by thepresent invention is sufficiently obtained by setting a wide deep guardring under the region. When an N+ electrode is formed even at a part ofthe deep guard ring region, carriers causing color mixture can finallyflow to the electrode. A reference numeral 1703 denotes N type regionforming a photodiode. A reference numeral 1704-4 denotes a concentrationprofile of a guard ring. A reference numeral 1706 denotes a poly siliconwiring. A reference numeral 1707 denotes n+ region of the MOStransistor.

This application claims priority from Japanese Patent Application No.2004-168575 filed on Jun. 7, 2004, which is hereby incorporated byreference herein.

1. An amplifying type image pickup device, comprising a plurality ofpixels, wherein each pixel includes: a photoelectric conversion unit forconverting incident light into a signal carrier; a floating diffusionportion, arranged adjacent to the photoelectric conversion unit so as tosandwich a portion of a transfer transistor between the floatingdiffusion portion and the photoelectric conversion unit, for convertingthe signal carrier generated by the photoelectric conversion unit into asignal voltage; an amplification transistor, having a gate connected tothe floating diffusion portion, for outputting an amplified signal basedon the signal voltage; a well of a second conductivity type arranged ona semiconductor substrate of a first conductivity type; a firstsemiconductor region of the first conductivity type, capable ofaccumulating the signal carrier, arranged in the well of the secondconductivity type and constituting a part of the photoelectricconversion unit; a second semiconductor region of the first conductivitytype, arranged in the well of the second conductivity type, operating asa source region or a drain region of the amplification transistor; anelement isolation region arranged between the first and secondsemiconductor regions; and a guard region portion of the secondconductivity type arranged under the element isolation region, the guardregion portion including a plurality of semiconductor regions of thesecond conductivity type respectively at different depths, having animpurity concentration higher than that of the well of the secondconductivity type, and extending to reach the semiconductor substrate,wherein the guard region portion of the second conductivity typesurrounds the first semiconductor region and the floating diffusionportion of the pixel, and the second semiconductor region is sandwichedbetween a guard region included in a first pixel and a guard regionincluded in a second pixel adjacent to the first pixel, so that the wellremains under the second semiconductor region.
 2. The image pickupdevice of claim 1, wherein the guard region portion is part of a guardregion of the second conductivity type, such that a deepest portion ofthe guard region of the second conductivity type is arranged under thesecond semiconductor region and between photoelectric conversion unitsof mutually adjacent first and second pixels.
 3. The image pickup deviceof claim 1, wherein the plurality of semiconductor regions included inthe guard region portion is arranged such that a width and an impurityconcentration increases as a depth increases.
 4. The image pickup deviceof claim 1, wherein the well includes a plurality of wells havingimpurity peaks.
 5. A camera comprising: an image pickup device thatincludes a plurality of pixels, wherein each pixel includes: aphotoelectric conversion unit for converting incident light into asignal carrier, a floating diffusion portion, arranged adjacent to thephotoelectric conversion unit so as to sandwich a portion of a transfertransistor between the floating diffusion portion and the photoelectricconversion unit, for converting the signal carrier generated by thephotoelectric conversion unit into a signal voltage, an amplificationtransistor, having a gate connected to the floating diffusion portion,for outputting an amplified signal based on the signal voltage, a wellof a second conductivity type arranged on a semiconductor substrateportion of a first conductivity type, a first semiconductor region ofthe first conductivity type, capable of accumulating the signal carrier,arranged in the well of the second conductivity type and constituting apart of the photoelectric conversion unit, a second semiconductor regionof the first conductivity type, arranged in the well of the secondconductivity type, operating as a source region or a drain region of theamplification transistor, an element isolation region arranged betweenthe first and second semiconductor regions, and a guard region portionof the second conductivity type arranged under the element isolationregion, the guard region portion including a plurality of semiconductorregions of the second conductivity type respectively at different depthshaving an impurity concentration higher than that of the well of thesecond conductivity type, and extending to reach the semiconductorsubstrate, wherein the guard region portion of the second conductivitytype surrounds the first semiconductor region and the floating diffusionportion of the pixel, and the second semiconductor region is sandwichedbetween a guard region included in a first pixel and a guard regionincluded in a second pixel adjacent to the first pixel, so that the wellremains under the second semiconductor region; a signal processingcircuit for processing a signal output from the image pickup device; andan optical member for imaging an object on a photoreceiving surface ofthe image pickup device.